Will2k can probably provide a more in-depth explanation than me, but in general, the higher your resolutions, the larger the map file and the lower the frame rates.
Will has an article here on MapCore that explains some of this.
I really like the assets you've made so far. I was not a fan of FMPONE's rendition of de_season because I thought the place looked entirely too clean and bright, but something about the way you've done it is pleasing to me... I think it may be because you've broken up the colors more so they're not as flat as Season's look.